No, that still requires some battery on iOS, i.e. it's only possible in the same "power reserve" mode that still sends the occasional "Find my iPhone" Bluetooth

No, that still requires some battery on iOS, i.e. it's only possible in the same "power reserve" mode that still sends the occasional "Find my iPhone" Bluetooth beacon. Field-powered mode is possible in at least some NFC chipsets, but I suspect that Apple either values a consistent NFC range more than usability even with a completely dead battery (the amplifier that grants a significantly higher NFC range to Apple Pay obviously needs power), they see it as a security feature (reserve mode is capped to a few hours, I believe), or their NFC controller simply doesn't support it.